@David-TP hi thanks for the reply. 

Yes I am describing modem-router with WiFi capacity. I am based in the UK and have a Virgin Media Home Hub model 3. 

Yes the PiHole is acting as a DHCP server and I have disabled the DHCP capability on the router. This set up worked fine before using the Deco units. 

The switch is a tp link 8 port 1Gig unit. 

I did try with a single node and the issue persisted. 

After doing some research on these forums I believe the issue might be that DHCP capabilities cannot be disabled on the Deco units even I'm AP mode. Can you confirm if this is the case?

I have worked around the issue by switching the Deco to function in router mode as I was quite pessimistic about being able to disable the Deco DHCP in AP mode. This is working fine as expected but isn't a great user experience for this with more advanced networks. 

  @David-TP are you able to respond to my question about the Deco DHCP server in AP mode?

  @AnnoyedUser69 

Deco mesh in AP mode will turn on its DHCP service when "primary" DHCP server on the network becomes unavailable.

As long as there is functioning DHCP server on your network Deco mesh can query, it (Deco) will not turn on its DHCP service, when running in AP mode.

  @Alexandre. Ok thanks for confirming. That didn't seem to be the behaviour I observed - there were IP issues as soon as I powered on the Deco units despite the primary DHCP server being active. I also saw other users reporting similar functionality. Is there any known unusual behaviour when the Deco units first power on? Do the Deco units expect the DHCP server to be the router or something like that?

  @AnnoyedUser69 

Judging from your other post and network diagram and problem description, you might have issues with wired part of your home network that could make DHCP service on Pi inaccessible to Main Deco. Even if briefly. That would be enough to trigger start of DHCP service on Deco.

Check configuration settings on your primary DHCP server. It may have setting responsible for "do not run (do not start) DHCP service if another DHCP server is active on network."

Turn that off, making it "always run DHCP service." 

That may reduce time Deco runs its DHCP service if your primary DHCP server is only briefly unavailable.

  @Alexandre. Thanks for the reply. I followed the advice from my other post, and after powering off the problem Deco (node 2) AND unplugging and plugging back in the switch ethernet cable, the network does now appear as expected.

It is interesting as there does not appear to be a fault with the cable itself or the switch, and when I first set the network up a few days ago it did appear correctly as well (before subsequently "breaking" and appearing as described in my other post). I am monitoring the network now to see if it does "break" again.

Do you know of any network monitoring tools that could help diagnose local connections across the network? I would be interested to see if there was any issues across the wired devices